The objective of this study is to characterize host-viral protein interactions involved in the expression of viral genes using coliphage Lambda and its E. coli host as a model system. The major emphasis will be directed towards analyzing the multiple factors involved in the regulation of gene expression by the N. transcription antitermination protein of Lambda. In addition to N, the components include the bacterial Nus proteins and a region on the phage genome, nut. We propose to continue our study of mutations that alter the activity of each of the components. Other possible components will be studied by characterizing mutations that suppress the effects of nus mutations. In particular, we will correlate structural differences with functional activity of the nusA gene and its product by analyzing: 1) conditionally lethal nusA mutations, 2) nusA genes from organisms other than E. coli, 3) hybrid E. coli-S. typhimurium nusA genes constructed in vitro, 4) truncated nusA proteins constructed by introducing chain terminating condons. The nut site will be studied to determine how sequence changes can create more optimal interactions with the component proteins. Mutations eliminating transcription termination will be analyzed to identify important elements in termination signals. A second set of E. coli mutations, sip, that influence the action of the lambdoid phage P22 cl transcription regulatory protein will be analyzed. The bacterial genes will be cloned and the proteins identified. Phage mutations that allow a bypass of the host block will be analyzed in order to determine the basis of the Sip defect. Studies on this well characterized system have significantly contributed to studies on the regulation of E. coli ribosomal RNA and biosynthetic operons. In the future they should also contribute to the study of other host-viral interactions such as those involved in malignant transformation.